Method for induction surface hardening of a ring surface

ABSTRACT

The invention relates to a method for the induction surface hardening of a ring surface of a workpiece, the ring surface and a hardening device having an induction coil being moved relative to one another in a treatment direction in a feed mode. A sprinkler is located downstream of the induction coil when seen in the direction of treatment and serves to heat the ring surface, starting from an initial zone to a final zone, by means of the induction coil and then to harden it by cooling the ring surface by means of the sprinkler, an unhardened soft zone being provided between the initial zone and the end zone. According to the invention, the supplementary induction coil, the induction coil and the sprinkler are activated and deactivated separately from one another at the start of the hardening process and at the end of the hardening process, respectively, the speed between the hardening device and the workpiece and/or the power supplied to the supplementary induction coil and the induction coil being varied at the same time.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Entry of International PatentApplication Serial Number PCT/EP2013/052346, filed Feb. 6, 2013, whichclaims priority to German National Patent Application Serial NumberDE102012101309.8, filed Feb. 17, 2012.

FIELD

The present invention relates to a method for induction surfacehardening of a ring surface of a workpiece, particularly a runningsurface of a bearing ring, the ring surface and a hardening device withan inductor being moved relative to one another in a direction oftreatment in a feeding mode, a spray being located downstream of theinductor, as seen in the direction of treatment, in order to heat thering surface from an initial zone to an end zone by means of theinductor and then harden it by cooling by means of the spray, anunhardened slip zone being provided between the initial zone and the endzone.

BACKGROUND

In the hardening of a closed curve trace in the form of a ring surfaceby local heating and subsequent quenching, there is the problem thatalready hardened regions must not be heated again to a temperature atwhich the hardness properties are lost again. In the case of a ringsurface, an unhardened slip zone, which has for example a width ofbetween 10 mm and 20 mm, is therefore provided between an initial zone,at which the hardening is begun, and an end zone, which is hardenedlast. In the case of rolling bearings, it is known to relief-grind thisslip zone, in order that the rolling bodies do not have bearing contactthere. It must also be taken into account that the initial zone and theend zone represent a transitional region with respect to the effectivedepth of hardening and the hardness at the surface, because bothparameters respectively decrease in the direction of the slip zone. Inthe hardened region between the initial zone and the end zone, on theother hand, uniform hardness properties are achieved as a result of auniform movement between the hardening device and the workpiece and alsoas a result of constant parameters. Apart from the speed between thehardening device and the ring surface, the hardening result can be setby the power output and the frequency of the inductor, the amount andtemperature of the coolant delivered by the spray and also bycontrolling any preheating that may be provided.

In the case of the methods for induction surface hardening with aninductor that are known from practice, the hardening device is activatedin the initial zone with the parameters intended for continuoushardening and is deactivated again in the end zone.

The documents DE 10 2005 006 701 B3, DE 10 2006 003 014 B1 and DE 102008 033 735 A1 disclose methods for induction surface hardening of aring surface of a workpiece. In the case of this method, a slip zone isavoided or reduced by using two inductors, which are moved counter toone another, starting from an initial zone. The initial zone can in thiscase be hardened completely, because uniform heating, or at leastlargely uniform heating, can be achieved by the two inductors. The twoinductors are then moved counter to one another and finally cometogether at the end zone, which on the ring surface lies opposite theinitial zone. The end zone is also heated only once, when the twoinductors approach one another from both sides. Since the two inductorscannot be brought up unrestrictedly close to one another, the end zonemay be initially preheated with a supplementary inductor. A disadvantageof the methods described is that, particularly in the case of largerolling bearings, very considerable expenditure is necessary with regardto the hardening device with counter-running inductors.

SUMMARY

The present invention is based on the object of providing a method withthe features described at the beginning in which the weakening of thering surface as a result of the reduced hardness in the initial zone andthe end zone is less pronounced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described in detail below with reference tothe attached drawing figures, wherein:

FIG. 1 is a side partial cross-section view of a region of an annularrunning surface of a workpiece at which an induction surface hardeningoperation begins and ends, and further depicting surface hardeningpenetration depths at an initial treatment zone and an end treatmentzone, with an unhardened slip zone remaining between the initial zoneand the end zone, after undergoing a surface hardening treatment methodas disclosed herein.

FIG. 2 is a side view of an embodiment of an apparatus for inductionsurface hardening of a work piece, as disclosed herein.

FIG. 3 is an embodiment of an exemplary change over time of varioushardness process parameters, according to an embodiment of the methodsdisclosed herein.

FIGS. 4a-4c are side partial schematic representations showingembodiments of temperature distribution patterns through a workpiecebeing subjected to a surface hardening process as disclosed herein,taken at different points in time at the beginning of the hardeningprocess.

DETAILED DESCRIPTION

On the basis of a method according to the preamble of patent claim 1,the object is achieved according to the invention by providing that, insuccession, a supplementary inductor, which is intended as a preheaterand is arranged upstream of the inductor in the direction of treatmentas a component part of the hardening device, is subjected to analternating current while the inductor and the spray are inactive; withthe supplementary inductor active and the inductor inactive and also thespray inactive, the hardening device is moved in relation to theworkpiece at a first speed; with the supplementary inductor still activebut the spray inactive, the inductor is activated by being subjected toan alternating current and a second speed is set; with the inductoractive and the supplementary inductor active, the spray is activated anda third speed is set; with the inductor active and the spray active, thesupplementary inductor is deactivated and a fourth speed is set and,finally, the inductor is deactivated.

The present invention is based on the realization that, in the case ofinduction surface hardening with an inductor, an unhardened slip zonecannot be avoided, but an increase in the hardness and the effectivedepth of hardening that is as quick as possible can be achieved byvariably controlling the hardness parameters, such as the speed, thepower output and the amount of spray, starting from the slip zone. Theinitial zone and the end zone, in which the hardness propertiesrespectively change, starting from the unhardened slip zone, untilsubstantially constant parameters are obtained, can in this way be keptas short as possible. The hardness parameters are changed in the initialzone, that is say at the beginning of hardening, in a number of steps,in order to achieve an increase in the hardness that is as great aspossible. It must be taken into account here that, before the beginningof hardening, the ring surface to be hardened is initially at ambienttemperature, for which reason the heat input by the supplementaryinductor as a preheater and also by the main inductor must be increasedor the speed must be reduced. A temperature gradient that is as steep aspossible is expediently set, in order also to achieve an increase in thehardness or the effective depth of hardening at the ring surface that isa steep as possible over a path that is as short as possible, startingfrom the slip zone. The hardness parameters are also changed in a numberof steps toward the end of the hardening operation, in order to keep theresultant heat input of the inductors initially as constant as possibleand also to achieve a temperature gradient that is as steep as possiblewith respect to the slip zone.

According to the invention, the hardness parameters are changed overtime in order to achieve an optimum result of the hardening operation atthe beginning of the hardening and at the end of the hardening. For thispurpose, particularly the power outputs supplied, the feeding speedand/or the amount of spray for the cooling are varied. Variation takesplace in a number of steps, it being intended in this way for continuouschanging of the hardness parameters over a time interval or over theentire hardening operation also to be included within the scope of theinvention. The previously defined speeds that are respectively set atthe beginning of a new phase then form as it were interpolation pointsof a progression that is changing continuously over time.

Within the scope of the invention, a relative movement between thehardening device and the ring surface is produced, the speeds relatingto the relative speed between the hardening device and the ring surface.It is thus possible for example to move the ring surface of theworkpiece past the fixed hardening device or else to move the hardeningdevice along the surface to be hardened with a workpiece at rest. Thefirst variant has proven to be successful particularly in the case ofrunning surfaces of a large rolling bearing, it being possible for thebearing ring to be arranged horizontally, vertically or at an angle andthe movement of the bearing ring taking place by way of supportingrollers. The direction of feeding of the bearing ring is then counter tothe direction of treatment in which the hardening operation proceedsduring the treatment.

According to the invention, firstly the preheater is activated, in orderinitially to preheat the ring surface in the initial zone. Since thering surface is initially cold with respect to the temperature intendedfor the hardening, the hardening device and the ring surface mayinitially remain at rest after the activation of the supplementaryinductor, and so the speed is equal to zero. Alternatively, there isalso the possibility of firstly setting a negative speed directly afterthe activation of the supplementary inductor. A relative movementbetween the ring surface and the hardening device that is counter to themovement during the continuous hardening operation then takes place.Thus, the supplementary inductor and also the inductor may be formed bya conductor loop with two parallel conductors. In order then to producea temperature gradient that is as steep as possible, a rearward movementmay take place between the ring surface and the hardening device, thepath of this rearward movement corresponding to half the distancebetween the two parallel conductors.

After a first preheating of the ring surface in the initial zone, thering surface is turned further, until the entire portion preheated atthe beginning lies under the inductor. The regions of the ring surfaceadjoining this first portion are in this case likewise preheated by thefurther movement at the first speed. Then, the inductor is activated bybeing subjected to alternating current, in order to bring the previouslypreheated region of the ring surface to the temperature intended for thehardening. The path covered after the activation of the supplementaryinductor and before the activation of the inductor expedientlycorresponds to the distance between these two devices. The second speed,set when activating the inductor, may in principle be equal to the firstspeed. Preferably, however, a higher speed is set, in order to makeallowance for the heating that has already taken place and thealtogether increased power output supplied.

In order finally to quench the ring surface heated up by the inductor,and thereby harden it, the spray arranged downstream of the inductor inthe direction of movement is activated with a delay. In principle, it isalso within the scope of the invention to vary the amount of spray, thatis to say the amount of cooling fluid supplied to the spray, accordingto requirements between the beginning of the hardening and the finalhardening.

With the activation of the spray, a third speed is set, which however ispreferably equal to the second speed.

After the activation of the spray, substantially constant conditions areestablished for the hardening and are used for hardening the greatestpart of the ring surface, for example at least 80% of the ring surface.In the course of setting virtually constant values, a further, optimumspeed may also be chosen for the hardening under constant conditions,such a fifth speed preferably being greater than the third speed.

In addition or as an alternative to the changing of the speeds, thepower output of the inductor or of the supplementary inductor may alsobe changed, the power output preferably being increased in an intervalfollowing the respective activation thereof as compared with an intervalfollowing thereafter. Such a timed increase in the power output at thebeginning makes allowance for the fact that constant conditions have notyet been obtained with respect to the temperature distribution.

A variation of the hardness parameters in a number of steps is alsointended in the region of the end zone. Thus, firstly the supplementaryinductor arranged upstream in the direction of treatment is deactivated,in order to avoid undesired heating in the region of the slip zone. Aspreviously explained, a temperature gradient that is as steep aspossible is also produced in the end zone, in order to make acorrespondingly steep transition between the hardened regions and theunhardened regions possible. When the deactivation of the supplementaryinductor occurs, a fourth speed is set, which is preferably reduced ascompared with the previously described fifth speed and may for examplecorrespond to the third speed. This reduction of the speed with respectto the substantially continuous hardening operation makes allowance forthe fact that the slip zone that has cooled down again represents a heatsink, requiring that the then absent heat input by the supplementaryinductor is also compensated to a certain extent. In order to increasethe heating by the inductor itself, in addition or alternatively thepower output of the inductor may also be increased. As already explainedabove, both measures, that is to say adaptation of the power outputsupplied and changing of the speed, may also be combined in a suitableway over the entire hardening operation.

Finally, the inductor is also deactivated, here too the path coveredbetween the deactivation of the supplementary inductor and the inductorcorresponding approximately to the distance between the supplementaryinductor and the inductor. The spray initially remains active, in orderto be able to quench the region of the end zone last heated up by theinductor. After the deactivation of the spray, the workpiece with thehardened ring surface can be removed from the hardening device.

Within the scope of the present invention, the distinction between theinductor and the supplementary inductor relates to the differentfunction of these two devices, that is to say the preheating and theheating up to the hardening temperature. The inductor and thesupplementary inductor may be similar or even identical in theirconfiguration. Suitable, for example, is a conductor loop with twoconductors parallel to one another that are connected by way of a leg.

Such a conductor loop is connected to an alternating current generator,which is operated for example at a frequency of between 3 kHz and 8 kHz.On account of the currents necessary for the heating, the conductor loopmay also be hollow and flowed through by a cooling medium.

In order to make precise control possible, particularly in the case oflarge workpieces, for example a bearing ring of a large rolling bearing,a reference point for controlling the hardening device may be determinedby a marking on the ring surface and a sensor assigned to the hardeningdevice. In this way, any dimensional deviations of the workpieces,inaccuracies in the driving of the workpiece or the hardening device andalso different thermal expansions cannot have an adverse influence onthe hardening process.

FIG. 1 shows by way of example the region of a ring surface 1 of aworkpiece 2, for example a running surface of a bearing ring, a slipzone 5 that is unhardened remaining between an initial zone 3, in whichthe induction surface hardening was begun, and an end zone 4, in whichthe induction surface hardening was ended. It is indicated that in theinitial zone 3 and the end zone 4 the effective depth of hardening andthe hardness respectively decrease in the direction of the slip zone 5.

With respect to the ring surface 1, the slip zone 5 represents aweakening and may for example be relief-ground, in order that rollingbodies do not have local bearing contact there. In addition, however, itis also evident that the surface hardness and the effective depth ofhardening are also reduced in the region of the initial zone 3 and theend zone 4 and respectively increase continuously from the unhardenedslip zone 5. Altogether, improved properties of the ring surface areachieved if not only the slip zone 5 is as short as possible, but alsothe effective depth of hardening and the surface hardness increase asquickly as possible within the initial zone 3 and the end zone 4, fromthe slip zone 5, to a value that is otherwise virtually constant aroundthe circumference.

In order to achieve this, the present invention provides a specificsetting of the hardness parameters in a number of steps at the beginningand the end of the hardening.

FIG. 2 shows an apparatus for induction surface hardening, the workpiece2 being vertically aligned and supported on rollers 6. By means of thedriven rollers 6, the outer lying ring surface 1 of the workpiece 2 canbe moved with respect to a fixed hardening device 7, the direction oftreatment B in which the hardening proceeds being counter to thedirection of feeding S of the workpiece 2.

The hardening device 7 comprises one behind the other, as seen in thedirection of feeding S, a supplementary inductor 8 as a preheater, aninductor 9 and a spray 10. In the exemplary embodiment represented, thehardening device 7 also comprises an optional sensor 11, in order todetect a marking 12 applied to the workpiece 2 and thus control thehardening operation.

FIG. 3 shows by way of example the change over time of various hardnessparameters during the hardening operation. The speed v between thehardening device 7 and the ring surface 1, the power output P_(H)supplied to the supplementary inductor 8 in the form of alternatingcurrent, the power output P_(I) supplied to the inductor in the form ofalternating current and the throughflow D of the coolant supplied to thespray 10 are represented.

Firstly, the workpiece 2 is turned at a high speed v₀, until the marking12 is sensed by the sensor 11. The workpiece 2 is then turned stillfurther at a reduced speed v₀′, until at the beginning of a first phaseI the supplementary inductor 8 intended as a preheater is activated andsubjected to the power output P_(H). In this first phase I, theworkpiece 2 remains at rest.

In the subsequent second phase II, it is still just the supplementaryinductor 8 that is active, the ring surface 1 being moved in the secondphase II at a first speed v₁ in a way corresponding to the distancebetween the supplementary inductor 8 and the inductor 9.

At the beginning of a third phase III, the inductor 9 is then activatedand subjected to a power output P_(I), an increased, second feedingspeed v₂ being set at the same time.

In a further phase IV, the previously inactive spray 10 is thensubjected to a predetermined throughflow D of coolant. It is only by thecooling of the ring surface 1 previously heated up by the supplementaryinductor 8 and the inductor 9 that a transformation of the material, andconsequently a surface hardening, takes place. With the activation ofthe spray, a third speed v₃ is set, which however is equal to the secondspeed v2. After the step-by-step activation of the supplementaryinductor 8, the inductor 9 and the spray 10, approximately constantconditions are established, but an increased, fifth feeding speed v₅being set in a further, fifth phase. During this fifth phase V, thegreatest part of the ring surface 1 is hardened under virtually constantconditions.

A number of steps are also provided in the ending of the hardeningoperation. Thus, firstly, at the beginning of a sixth phase VI, thesupplementary inductor 8 is deactivated. At the same time, a fourthspeed v₄ is set, which in the exemplary embodiment corresponds to thefirst speed v₁. The reduction of the speed makes allowance for the factthat heating of the ring surface 1 can then only take place by theinductor 9, and no longer by the supplementary inductor 8, for whichreason the lower energy input is equalized by a correspondingly reducedfeed.

Finally, at the beginning of a seventh phase VII, the inductor 9 is alsodeactivated. The spray 10 in this case initially remains active with areduced throughflow D′, in order to cool down the last-heated region ofthe ring surface 1 in the end zone 4. At the beginning of an eighthphase VIII, finally, the spray 10 is also completely deactivated, and sothe hardening operation is completely ended.

FIGS. 4a to 4c show by way of example the temperature distribution atthe hardening device during the first phase I, at the beginning of thethird phase II and also during the fifth phase V. The temperatureprogression is in this case indicated by equipotential lines (lines ofequal temperature).

According to FIG. 4a , the supplementary inductor 8, as a preheater,brings about local heating, the workpiece 2 still remaining at restduring the first phase I. During the second phase II, the workpiece 2 isthen moved in the direction of feeding S, whereby the region previouslypreheated under the supplementary inductor 8 lies under the inductor 9.Only then is the inductor 9 activated at the beginning of the thirdphase III, in order to achieve a temperature sufficient for the surfacehardening (FIG. 4b ).

FIG. 4c finally shows the progressive hardening in the fifth phase Vunder virtually constant conditions, the ring surface 1 that is takenpast the hardening device 7 being continuously preheated by thesupplementary inductor 8, brought to the necessary temperature for thesurface hardening by the inductor 9 and finally quenched by the spray10.

The invention claimed is:
 1. A method for induction surface hardening a ring surface of a workpiece by a hardening device having (1) a primary heating inductor configured to heat the ring surface of the work piece from an initial zone to an end zone, (2) a preheater inductor disposed upstream of the primary heating inductor in a direction of treatment and configured to preheat the ring surface of the workpiece before the ring surface encounters the primary heating inductor, (3) a sprayer disposed downstream of the primary heating inductor in a direction of treatment and configured to quench the heated work piece so as to harden the surface of the workpiece from the initial zone to the end zone with an unhardened slip zone disposed between the initial zone and the end zone, the hardening device and the ring surface of the work piece being movable relative to one another in a direction of treatment when in a feeding mode, comprising: activating the preheater inductor, so as to preheat the ring surface of the work piece; setting a first speed v1 at which the hardening device and the work piece move relative to each other in a direction of treatment, so as to preheat locations on the ring surface moving past the preheater inductor; activating the primary heating inductor so as to increase the temperature of locations of the ring surface of the workpiece moving past the primary heating inductor, and simultaneously setting a second speed v2 at which the hardening device and the work piece move relative to each other in a direction of treatment, wherein the second speed v2 is greater than the first speed v1; activating the sprayer so as to apply quenching liquid to locations of the ring surface moving past the sprayer and quench the locations on the ring surface that have been heated by the primary heating inductor, and simultaneously setting a third speed v3 at which the hardening device and the work piece move relative to each other in a direction of treatment; deactivating the preheater inductor, and simultaneously setting a fourth speed v4 at which the hardening device and the work piece move relative to each other in a direction of treatment; and deactivating the primary heater inductor.
 2. The method of claim 1, further comprising maintaining an initial at-rest position of the work piece relative to the hardening device directly after activating the preheater inductor.
 3. The method of claim 1, further comprising setting a negative speed of relative motion between the work piece and the hardening device, by which the hardening device and work piece move relative to each other in a direction opposite the direction of treatment.
 4. The method of claim 1, wherein the second speed v2 is equal to the third speed v3.
 5. The method of claim 1, further comprising setting a fifth speed v5 at which the hardening device and the work piece move relative to each other in a direction of treatment, after said activating of the sprayer and before said deactivating the preheater inductor, wherein the fifth speed v5 is greater than the third speed v3 and the fourth speed v4.
 6. The method of claim 1, further comprising: marking a reference point on the ring surface of the work piece, the location of which is to be used as an input to control one or more operational parameters of the hardening device; sensing the marked reference point by a sensor of the hardening device; and controlling one or more operational parameters of the hardening device by the sensor upon said sensing the marked reference point. 